Asteroid Psyche may once have had spouts spewing molten metals, but only if they were chemically similar to rare metal-rich meteorites, a new study suggests. The study, published online July 31 in the Journal of Geophysical Research: Planets, could explain why space rocks have unusual metallic mantles.
A member of the major asteroid belt between Mars and Jupiter, the potato-shaped asteroid Psyche is unique in that it is extremely shiny. On average, its surface reflects nearly one-third of the sunlight that hits it, making it at least twice as reflective as most asteroids, according to radar measurements.
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However, this shine may be mostly skin-deep. In 2020, calculations including the latest estimates of Psyche’s mass and volume suggested that the asteroid’s density is between 231 pounds per cubic foot (3,700 kilograms per cubic meter) and 256 pounds per cubic foot (4,100 kilograms per cubic meter). Although denser than most asteroids, it is only about half as dense as would be expected if Psyche were made entirely of iron and nickel. This, together with surface heat emission data, showed that while Psyche is largely non-metallic, the asteroid likely has a metal-rich mantle.
Scientists still do not know how Psyche got this coat. Many phenomena may be responsible, but iron volcanism is currently the most likely cause. “Iron volcanism is similar to normal volcanism, except that the ‘lava’ is molten metal rather than molten rock,” said Professor Courville, who was not involved in the new study.
The idea, first proposed in 2019, is that during Psyche’s childhood, her metal core solidified from the outside in, with the molten inner core gradually becoming richer in lighter elements. The difference in density between the core’s solid exterior and molten interior would have created enough pressure for the latter to pierce the overlying mantle and surface of the asteroid, creating a vent that spewed out molten metal.
However, strong volcanism only works with certain chemical compositions. This is because it largely depends on how much pressure builds up in the inner core. Data from meteorites (space rocks that have fallen to Earth’s surface) can help limit the range of these chemical combinations.
Dr. Jaap Jorritsma, author of the new study, conducted research to identify the chemicals that may have produced the iron vents at Psyche. Win van Westrenen, a student at Delft University of Technology and professor of geosciences at Vrije University in Amsterdam, created a computer model of the asteroid.
They thought the asteroid had a chemical structure similar to one of three types of meteorites. These are EH chondrites (rare stony meteorites lacking iron), H chondrites (common stony meteorites containing moderate amounts of iron), and mesosiderites (rare iron-rich meteorites). The researchers then ran simulations to figure out which chemical compositions were most suitable for the occurrence of iron volcanism.
The researchers found that Psyche’s bulk must be rich in metals, such as iron, because the internal pressure created by low iron content causes a small core that is insufficient to push out the magma. This meant that if Psyche contained large amounts of mesosiderite, it most likely had iron volcanic activity. H-chondrites can also cause eruptions of molten metal, but only if their densities are relatively high. In contrast, the EH chondrite produced only a small core without an iron-rich exterior and was therefore unlikely to form part of Psyche.
Researchers hope NASA’s ongoing Psyche mission will provide evidence to support their findings. The spacecraft is scheduled to arrive at Psyche by July 2029, and will spend two years photographing the asteroid and collecting spectroscopic data. Courville said these snapshots will reveal whether the metal on the surface is in large outcrops or flows, indicating that strong volcanic activity occurred in the space rock’s past.
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